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</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><strong class="selflink"> Preprocessor</strong><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/conditional.html" title="c/preprocessor/conditional"><span class="t-lines"><span>#if</span><span>#ifdef</span><span>#ifndef</span><span>#else</span><span>#elif</span><span>#elifdef</span><span>#elifndef</span><span>#endif</span></span></a></div><div><span class="t-lines"><span></span><span></span><span></span><span></span><span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span><span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/replace.html" title="c/preprocessor/replace"><span class="t-lines"><span>#define</span><span>#undef</span><span>#,## operators</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/include.html" title="c/preprocessor/include"><span class="t-lines"><span>#include</span><span>__has_include</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/error.html" title="c/preprocessor/error"><span class="t-lines"><span>#error</span><span>#warning</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c23">(C23)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../cpp/preprocessor/impl.html" title="cpp/preprocessor/impl"><span class="t-lines"><span>#pragma</span><span>_Pragma</span></span></a></div><div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/line.html" title="c/preprocessor/line"><span class="t-lines"><span>#line</span></span></a></div></div></td></tr>
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<p>The preprocessor is executed at <a href="language/translation_phases.html" title="c/language/translation phases">translation phase 4</a>, before the compilation. The result of preprocessing is a single file which is then passed to the actual compiler.
</p>
<h3><span class="mw-headline" id="Directives">Directives</span></h3>
<p>The preprocessing directives control the behavior of the preprocessor. Each directive occupies one line and has the following format:
</p>
<ul><li> <code>#</code> character
</li><li> preprocessing instruction (one of <code>define</code>, <code>undef</code>, <code>include</code>, <code>if</code>, <code>ifdef</code>, <code>ifndef</code>, <code>else</code>, <code>elif</code><span class="t-rev-inl t-since-c23"><span>, <code>elifdef</code>, <code>elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>, <code>endif</code>, <code>line</code><span class="t-rev-inl t-since-c23"><span>, <code>embed</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>, <code>error</code><span class="t-rev-inl t-since-c23"><span>, <code>warning</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>,  <code>pragma</code>) <sup id="cite_ref-1" class="reference"><a href="preprocessor.html#cite_note-1">[1]</a></sup>
</li><li> arguments (depends on the instruction)
</li><li> line break
</li></ul>
<p>The null directive (<code>#</code> followed by a line break) is allowed and has no effect.
</p>
<h3><span class="mw-headline" id="Capabilities">Capabilities</span></h3>
<p>The preprocessor has the source file translation capabilities:
</p>
<ul><li> <b><a href="preprocessor/conditional.html" title="c/preprocessor/conditional"> conditionally</a></b> compile of parts of source file (controlled by directive <code>#if</code>, <code>#ifdef</code>, <code>#ifndef</code>, <code>#else</code>, <code>#elif</code><span class="t-rev-inl t-since-c23"><span>, <code>#elifdef</code>, <code>#elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> and <code>#endif</code>).
</li><li> <b><a href="preprocessor/replace.html" title="c/preprocessor/replace"> replace</a></b> text macros while possibly concatenating or quoting identifiers (controlled by directives <code>#define</code> and <code>#undef</code>, and operators <code>#</code> and <code>##</code>)
</li><li> <b><a href="preprocessor/include.html" title="c/preprocessor/include">include</a></b> other files (controlled by directive <code>#include</code>)
</li><li> cause an <b><a href="preprocessor/error.html" title="c/preprocessor/error">error</a></b> <span class="t-rev-inl t-since-c23"><span>or <b><a href="preprocessor/error.html" title="c/preprocessor/error">warning</a></b></span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span> (controlled by directive <code>#error</code><span class="t-rev-inl t-since-c23"><span> or <code>#warning</code> respectively</span> <span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>)
</li></ul>
<p>The following aspects of the preprocessor can be controlled:
</p>
<ul><li> <b><a href="preprocessor/impl.html" title="c/preprocessor/impl"> implementation defined</a></b> behavior (controlled by directive <code>#pragma</code><span class="t-rev-inl t-since-c99"><span> and operator <code>_Pragma</code></span> <span><span class="t-mark-rev t-since-c99">(since C99)</span></span></span>)
</li><li> <b><a href="preprocessor/line.html" title="c/preprocessor/line"> file name and line information</a></b> available to the preprocessor (controlled by directives <code>#line</code>)
</li></ul>
<h3><span class="mw-headline" id="Footnotes">Footnotes</span></h3>
<ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><a href="preprocessor.html#cite_ref-1">↑</a></span> <span class="reference-text">These are the directives defined by the standard. The standard does not define behavior for other directives: they might be ignored, have some useful meaning, or make the program ill-formed. Even if otherwise ignored, they are removed from the source code when the preprocessor is done. <span class="t-rev-inl t-until-c23"><span>A common non-standard extension is the directive <code>#warning</code> which emits a user-defined message during compilation.</span> <span><span class="t-mark-rev t-until-c23">(until C23)</span></span></span></span>
</li>
</ol>
<h3><span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-17">
<ul><li> C17 standard (ISO/IEC 9899:2018): 
</li></ul>
<dl><dd><ul><li> 6.10 Preprocessing directives (p: 117-129)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 6.10 Preprocessing directives (p: 160-178)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999): 
</li></ul>
<dl><dd><ul><li> 6.10 Preprocessing directives (p: 145-162)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990): 
</li></ul>
<dl><dd><ul><li> 3.8 PREPROCESSING DIRECTIVES 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

</table>

<!-- 
NewPP limit report
Preprocessor visited node count: 1815/1000000
Preprocessor generated node count: 5291/1000000
Post‐expand include size: 42683/2097152 bytes
Template argument size: 6519/2097152 bytes
Highest expansion depth: 16/40
Expensive parser function count: 0/100
-->

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